drm/i915: Move intel_engines_resume into common init

Since this part still operates on i915 and not intel_gt, move it to the
common (top-level) function.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-16-tvrtko.ursulin@linux.intel.com